Steel cord

ABSTRACT

There is provided a steel cord including a plurality of untwisted core filaments of steel aligned in parallel, and a layer of sheath filaments of steel twisted around the core filaments so as to be unevenly distributed around the core filaments, wherein interstices between the filaments are maintained during vulcanization thereby achieving improved rubber penetration (sufficiently adhering rubber to the core filaments). Since the cross sectional length of the steel cord  10  is greater than the minimum cross sectional length, interstices A are maintained between sheath filaments  14  under the tension and pressure p of the surrounding rubber  16  applied to the steel cord  10  during vulcanization. Rubber  16  penetrates into the steel cord  10  through the interstices A, and sufficiently adhere to core filaments  12  to achieve high rubber penetration.

TECHNICAL FIELD

The present invention relates to a steel cord, including a plurality ofuntwisted core filaments of steel aligned in parallel, and a layer ofsheath filaments of steel twisted around the core filaments so as to beunevenly distributed around the core filaments.

BACKGROUND ART

Steel cords for reinforcing rubber articles such as pneumatic tires havea variety of twisting structures. In order to achieve so-called rubberpenetration (easiness of penetration of rubber between filaments duringrubber coating), usually, the form of filaments is enlarged therebyproviding adequate interstices between filaments, or sheath filamentsare arranged around core filaments in a number slightly smaller than themaximum allowable number thereby providing adequate interstices.

Specifically, for example, Patent Document 1 discloses a steel cordincluding core filaments composed of a plurality of core wires alignedon the same level, and a plurality of side wires twisted around the corefilaments so as to form a flat cross section, wherein interstices areprovided between the core and side wires at the ends of the steel cordin the width direction.

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 2002-180387

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, in a steel cord composed of core filaments and sheath filamentstwisted around the core filaments not at regular intervals but in anunevenly distributed state, the untwisted core filaments aligned inparallel are pulled so as to be slightly undulated by the twistingtension of the sheath filaments. As a result of this, the core filamentsare brought into contact with the sheath filaments on the inside of thebending portion (compressed side).

In particular, in twisted portions wherein core filaments aligned inparallel in one direction are covered by sheath filaments in a directiongenerally perpendicular to the aligning direction, even if the filamentsare coated with rubber, the filaments are brought into contact with eachother to have no interstices between them by the tension applied duringvulcanization and the pressure of the surrounding rubber, which resultsin the formation of closed spaces containing no rubber (not penetratedby rubber) within the cord.

The present invention has been made to solve the above problems, and isintended to provide a steel cord including a plurality of untwisted corefilaments of steel aligned in parallel, and a layer of sheath filamentsof steel twisted around the core filaments so as to be unevenlydistributed around the core filaments, wherein interstices between thefilaments are maintained during vulcanization thereby achieving improvedrubber penetration (sufficiently attaching rubber to the corefilaments).

Means for Solving the Problem

In a steel cord including a plurality of untwisted core filaments ofsteel aligned in parallel, and a layer of sheath filaments of steeltwisted around the core filaments so as to be unevenly distributedaround the core filaments, in order to achieve good rubber penetrationinto the twisted portions wherein the core filaments aligned in parallelin one direction are covered by sheath filaments in a directiongenerally perpendicular to the aligning direction, interstices must bemaintained between the sheath filaments in the portions. In order toachieve this, the sheath filaments at both ends in the aligningdirection must be arranged with adequate clearance around them in themaximum width direction of the steel cord (but the sheath filaments maybe in contact with the core filaments). In the present description, thesectional length φ of the cord in a cross section shown in FIG. 2 ishereinafter referred to as “cross sectional length”.

The steel cord of the present invention includes two untwisted corefilaments, each having a diameter of d_(c), aligned in parallel, and alayer composed of four sheath filaments, each having a diameter ofd_(s), twisted around the core filaments so as to be unevenlydistributed around the core filaments, the cross sectional length φ inthe aligning direction of the core filaments satisfying the followingformula (1):

$\begin{matrix}{\varphi > {{2d_{s}} + \frac{{2{d_{s}^{2}\left( {d_{c} - d_{s}} \right)}} + {4d_{c}d_{s}\sqrt{d_{s}\left( {{2d_{s}} + d_{c}} \right)}}}{\left( {d_{c} + d_{s}} \right)^{2}}}} & (1)\end{matrix}$

The right-hand side of the formula (1) expresses the cross sectionallength of the steel cord wherein the filaments are arranged in closecontact with each other. The right-hand side is referred to as “minimumcross sectional length”.

In the steel cord of the present invention, the cross sectional length φis greater than the minimum cross sectional length expressed by theright-hand side of the formula (1), hence interstices are maintainedbetween sheath filaments under the tension and pressure of thesurrounding rubber applied during rubber coating and vulcanization ofthe steel cord, and the rubber penetrates through the interstices tosufficiently adhere to the core filaments. Consequently, the steel cordof the present invention achieves good rubber penetration.

The upper limit of the cross sectional length φ is 2d_(s)+2d_(c), whichis the sum of the diameters of two core filaments and two sheathfilaments at the both ends aligned in contact with each other.

In the present invention, the cross sectional length φ is preferably notsmaller than the right-hand side of the formula (1)+0.01 mm, and thediameter d_(s) of a sheath filament and the diameter d_(c) of a corefilament are preferably from 0.10 to 0.40 mm.

Advantages

As described above, the steel cord of the present invention includes aplurality of untwisted core filaments of steel aligned in parallel, anda layer of sheath filaments of steel twisted around the core filamentsso as to be unevenly distributed around the core filaments. The steelcord achieves markedly improved rubber penetration (sufficientlyadhering rubber to the core filaments) through the maintenance ofinterstices between filaments during vulcanization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plane view of a steel cord and cross sectional views ofrespective portions of the steel cord.

FIG. 2 shows a cross sectional view of a steel cord.

FIG. 3 shows a cross sectional view of a ribbon composed of steel cordscoated with vulcanized rubber.

FIG. 4 shows a schematic view of a tubular strander.

REFERENCE NUMERALS

10 steel cord

12 core filaments

14 sheath filaments

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described on the basis ofdrawings. As shown in FIGS. 1 and 2, a steel cord 10 according to anembodiment of the present invention includes two untwisted corefilaments 12, each having a diameter of d_(c) (mm), aligned in parallel,and a layer composed of four sheath filaments 14, each having a diameterof d_(s) (mm), twisted around the core filaments 12 so as to be unevenlydistributed around the core filaments 12, the cross sectional length φsatisfying the following formula (1):

$\begin{matrix}{\varphi > {{2d_{s}} + \frac{{2{d_{s}^{2}\left( {d_{c} - d_{s}} \right)}} + {4d_{c}d_{s}\sqrt{d_{s}\left( {{2d_{s}} + d_{c}} \right)}}}{\left( {d_{c} + d_{s}} \right)^{2}}}} & (1)\end{matrix}$

As described above, the right-hand side of the formula (1) expresses theminimum cross sectional length of the cord wherein the filaments arearranged in close contact with each other. Therefore, when the crosssectional length φ is greater than the minimum cross sectional length,interstices A can be formed between the sheath filaments 14. In order toachieve rubber penetration more reliably, the cross sectional length φis preferably greater than the minimum cross sectional length by 0.01 mmor more.

As described above, the upper limit of the cross sectional length φ is2d_(s)+2d_(c), which is the sum of the diameters of two core filaments12 and two sheath filaments 14 at the ends aligned in contact with eachother.

When the steel cord 10 of the present invention is used for reinforcinga tire, the diameter of the core filaments 12 and sheath filaments 14 ispreferably from 0.10 to 0.40 mm. If the filament diameter is too small,the filaments are disadvantageous costwise, and if too large, they havea low strength per unit weight due to insufficient work-hardening, andhave too high flexural rigidity to lack flexibility, and exhibit poorfatigue resistance against bending strain.

When the core filaments 12 and sheath filaments 14 have the samediameter, they offer a cost advantage. In this case, a layer of up toeight sheath filaments 14 can be twisted around the two core filaments12 arranged in parallel with each other. The rubber penetration isimproved by removing four sheath filaments 14, which results insufficient adherence of rubber 16 (FIG. 3) to the core filaments 12after vulcanization.

(Operation)

As shown in FIG. 3, in the steel cord 10, interstices A are maintainedbetween the sheath filaments 14, and the interstices A will not be losteven under the tension and pressure p of the surrounding rubber 16applied to the steel cord 10 during vulcanization. Therefore, the rubber16 penetrates into the steel cord 10 through the interstices A, andadheres to the core filaments 12.

As described above, the steel cord 10 of the present invention achievesgood rubber penetration with a structure including the sheath filaments14 twisted around the core filaments 12 so as to be unevenly distributedaround the core filaments 12. The use of the steel cord 10 allows themanufacture of rubber articles such as a ribbon 36 with sufficientrubber penetration.

The ribbon 36, which is composed of the steel cord 10 of the presentinvention embedded in rubber, is useful for, for example, making abelt-reinforcing layer of a tire (not shown). A belt-reinforcing layerincluding the ribbon 36 is resistant to entry of moisture into thelayer, specifically into the steel cord, even if a tread (not shown) iscut, and thus offers better corrosion resistance.

(Method and Apparatus for Producing Steel Cord)

The steel cord 10 of the present invention may be produced with, forexample, a tubular strander 20 shown in FIG. 4. In the tubular strander20, the core filaments 12 are reeled out from a plurality of corefilament bobbins 22, the sheath filaments 14 are reeled out from aplurality of sheath filament bobbins 26, which are contained in a rotarybarrel 24, and formed by a preformer 28, and then the core filaments 12and the sheath filaments 14 are assembled at the junction 30 to betwisted together. The twisted steel cord 10 is passed between thestraightening rolls 32, and wound around, for example, a reel 34. In thetubular strander 20, an appropriate tension is applied to the corefilaments 12 reeled out from the core filament bobbins 22.

In the tubular strander 20, the sheath filaments 14 reeled out from therotary barrel 24 are formed by the preformer 28 and sent to the junction30, at the same time, the core filaments 12 reeled out from the corefilament bobbins 22 outside the rotary barrel 24 are aligned in parallelin an untwisted state without being subjected to forming, and then sentto the center of the junction 30.

Since the rotary barrel 24 is rotating, the sheath filaments 14 aretwisted around the core filaments 12 at the junction 30 to form thesteel cord 10. The twisted steel cord 10 is straightened by thestraightening rolls 32, and wound around the reel 34.

The cross sectional length φ of the steel cord 10 is controlled bychanging the tension applied to the core filaments 12 before twisting,and changing the degree of bending of the steel cord 10 through thecontrol of the engagement between the upper and lower rolls of thestraightening rolls 32.

Specifically, for example, when the tension applied to the corefilaments 12 is decreased and the degree of bending of the steel cord 10at the straightening rolls 32 is increased, the steel cord 10 tends tobe rounded (the cross sectional length φ decreases) in the twistedportions wherein the core filaments 12 aligned in one direction arecovered by the sheath filaments 14 in a direction generallyperpendicular to the aligning direction.

The aligning direction is the direction along which the core filaments12 are aligned. For example, in FIG. 2, the lateral directioncorresponds to the aligning direction. The aligning direction of thecore filaments 12 is not limited to the lateral direction.

Examples

The present invention will be illustrated with reference to thefollowing examples.

The steel cords of Examples and Comparative Examples listed in Table 1were concurrently embedded in a periphery of a belt layer (the firstbelt layer located at the innermost part in the tire diameter direction)in a prototype tire having a tire size of 185/70R14 and twobelt-reinforcing layers. The steel cords were removed from the tireafter vulcanization, and the degree of adherence of the surface rubberto the core filaments after removal of the sheath filaments was observedthereby evaluating the rubber penetration. Regarding ComparativeExamples 1 and 2, the measured value of the cross sectional length φ wassmaller than the minimum cross sectional length (calculated value).

The evaluation of the rubber penetration rate is exclusively based onthe observation of cross sections of ten twisted portions wherein corefilaments aligned in parallel in one direction are covered by sheathfilaments in a direction generally perpendicular to the aligningdirection, and is expressed by the ratio (percentage) of cross sectionswhich achieved rubber penetration. The results are listed in Table 1.

TABLE 1 Example Example Comparative Example Comparative 1 2 Example 1 3Example 2 Twisted structure 2 + 4 2 + 4 2 + 4 2 + 4 2 + 4 Core filamentdiameter 0.225 0.225 0.225 0.23 0.23 d_(c) (mm) Sheath filament diameter0.225 0.225 0.225 0.21 0.21 d_(s) (mm) Twisting pitch (mm) 14 14 14 1414 Minimum cross sectional length 0.840 0.840 0.840 0.798 0.798Calculated value of the right-hand side of the formula (1) Measuredvalue of cross 0.854 0.844 0.831 0.814 0.785 sectional length φ (mm)Rubber penetration rate (%) 90 50 30 80 30

As is evident from the results in Table 1, the measured value of thecross sectional length φ of Comparative Examples 1 and 2 was smallerthan the minimum cross sectional length (calculated value), so thattheir rubber penetration rate was as low as 30%. On the other hand, themeasured value of the cross sectional length φ of Examples 1 to 3 wasgreater than the minimum cross sectional length (calculated value), sothat their rubber penetration rate was greater than that of ComparativeExamples. In particular, the measured value of the cross sectionallength φ of Examples 1 and 3 was greater than the minimum crosssectional length (calculated value) by 0.01 mm or more, so that theirrubber penetration rate was markedly high.

1. A steel cord comprising two untwisted core filaments, each having adiameter of d_(c), aligned in parallel, and a layer composed of foursheath filaments, each having a diameter of d_(s), twisted around thecore filaments so as to be unevenly distributed around the corefilaments, the cross sectional length φ satisfying the following formula(1): $\begin{matrix}{\varphi > {{2d_{s}} + \frac{{2{d_{s}^{2}\left( {d_{c} - d_{s}} \right)}} + {4d_{c}d_{s}\sqrt{d_{s}\left( {{2d_{s}} + d_{c}} \right)}}}{\left( {d_{c} + d_{s}} \right)^{2}}}} & (1)\end{matrix}$
 2. The steel cord according to claim 1, wherein the crosssectional length 4 is not smaller than the right-hand side of theformula (1)+0.01 mm.
 3. The steel cord according to claim 1, whereind_(s), and d_(c) are from 0.10 to 0.40 mm.
 4. The steel cord accordingto claim 2, wherein d_(s) and d_(c) are from 0.10 to 0.40 mm.